There are lots of examples of a porous structure made by using various materials and production methods. The porous structure is used for products in a wide range of fields including optical elements such as low refractive index layers, heat insulating materials, sound absorbing materials, and regenerative medical bases. The porous structure includes a closed-cell structure in which independent void spaces (pores) are dispersed, an open-cell structure in which the closed-cell structures are interconnected, and the like, which are defined according to the dispersion state of pores. The porous structure can be defined also according to a size of the void space and various other matters.
As a method of producing such a porous structure, for example, there is a method of substituting a solvent contained in a wet gel with gas under its supercritical condition to obtain a dry gel with no shrinkage in which the skeletal structure of the wet gel is frozen as it is (see, for example, Patent Document 1). This dry gel can be divided into: a xerogel obtained by gradually removing a gel solvent by evaporation under a normal pressure; and an aerogel, which is “a gel like air”, having a low bulk density and a high porosity.
A common problem in producing an aerogel bulk body is to prevent a gel body from cracking in drying of the gel. The crack is made when the tensile stress by the capillary force based on the surface tension of a solution remaining in a pore of the gel body in drying is greater than the strength of the gel. Under a supercritical condition, a bulk body with no crack can be obtained because of no surface tension, however, there is a case that a crack is made during high temperature sintering treatment as a process of removing big pores afterwards. For the purpose of reducing cracks to be caused during such high temperature treatment, there are examples of using a solvent having a higher boiling point than water and having a small surface tension, mixing silica fine particles into a solvent, and the like (Non-Patent Document 1).
On the other hand, forming a silicone porous body having a high proportion of void space (porosity) has a problem that its strength significantly decreases because of a decrease in the bulk density of a silica gel material. The decrease in the strength causes a problem in use, such as a decrease in an abrasion resistance. Regarding this matter, there have been disclosed the methods of baking a silicone porous body to increase the strength (see, for example, Patent Documents 2 to 5). These methods, however, are premised on a batch process since high temperature treatment at 200° C. or more is performed for a long period of time in baking treatment. Thus, a continuous production cannot be performed industrially. Moreover, the baking treatment has a problem of causing cracks because of the great change in volume in cooling upon completion of the sintering after the crystal stable phase of a silica gel has been transited from a low-temperature phase to a high-temperature phase.
On the other hand, there has also been disclosed a method of applying alkali treatment to a silica aerogel film to cause a condensation reaction of an unreacted silanol group, thereby increasing the strength of the silica aerogel film (see, for example, Patent Document 6). In this case, however, since a formed silica aerogel film is immersed in an alkali treatment solution, a dehydration condensation reaction of the unreacted silanol group due to penetration of the alkaline solution into voids is assumed to be caused. This causes a silica aerogel film to be swollen and dried, and this decreases the proportion of void space of the silica aerogel film after the alkali treatment. The strength and the proportion of void space have a trade-off relationship and it is difficult to achieve both properties.